Group III nitride containing crystalline materials serve as substrates for manufacture of optoelectronic devices such as violet or blue light emitting diodes and laser diodes and electronic devices such as diodes and switches. Such optoelectronic and electronic devices have been commonly manufactured on sapphire, silicon, or silicon carbide substrates that differ in composition from the deposited nitride layers. The use of group III nitride substrates can provide a number of benefits, including increased performance and lifetime.
Most bulk GaN substrates today are manufactured by hydride vapor phase epitaxy (HVPE), a vapor phase technique that deposits a thick layer of GaN on a substrate, typically sapphire or gallium arsenide. To date, HVPE substrates have been relatively expensive, and the crystalline quality may be insufficient for certain applications, such as high performance laser diodes or vertical GaN-on-GaN power electronic devices.
Superior crystalline quality has been demonstrated by true bulk crystal growth techniques, including ammonothermal crystal growth. Ammonothermal crystal growth methods are expected to be scalable, as described by Dwilinski, et al. (J. Crystal Growth 310, 3911 (2008)), by Ehrentraut, et al. (J. Crystal Growth 305, 204 (2007)), by D'Evelyn, et al. (J. Crystal Growth 300, 11 (2007)), and by Wang, et al. [Crystal Growth & Design 6, 1227 (2006)]. The ammonothermal method generally requires a polycrystalline nitride raw material, which is then recrystallized onto seed crystals.
An ongoing challenge of certain ammonothermally-grown GaN crystals is a significant level of impurities, which cause the crystals to be colored, e.g., yellowish, greenish, grayish, or brownish. The residual impurities may cause optical absorption in light emitting diodes fabricated on such substrates, negatively impacting efficiency, and may also affect the electrical conductivity and/or generate stresses within the crystals. One source of the impurities is the polycrystalline group III nitride raw material. In addition, the polycrystalline GaN nutrient must be inexpensive if the GaN crystals grown from it are to be inexpensive, and therefore the synthesis process must be scalable to large volumes, efficient, and cost-effective.
Several methods for synthesis of polycrystalline group III nitride materials have been proposed. Callahan, et al. [MRS Internet J. Nitride Semicond. Res. 4, 10 (1999); U.S. Pat. No. 6,406,540] proposed a chemical vapor reaction process involving heating gallium metal in a vapor formed by heating NH4Cl. Related conversion-in-place methods have been discussed by Wang, et al. [J. Crystal Growth 286, 50 (2006)], by Callahan, et al., [U.S. Pat. No. 8,858,708] and by Park, et al. [U.S. Application Publication Nos. 2007/0142204, 2007/0151509, and 2007/0141819]. In the case of Park, the predominant impurity observed was oxygen, at levels varying from about 16 to about 160 parts per million (ppm). The chemical form of the oxygen was not specified. An alternative method, involving heating in ammonia only and producing GaN powder with an oxygen content below 0.07 wt %, was disclosed by Tsuji (U.S. Publication No. 2008/0193363). Yet another alternative method, involving contacting Ga metal with a wetting agent such as Bi and heating in ammonia only, producing GaN powder with an oxygen content below 650 ppm, has been disclosed by Spencer, et al. (U.S. Pat. No. 7,381,391). Other methods involve downstream synthesis of polycrystalline GaN, that is, where the material is deposited downstream from a source of gallium metal. Examples of downstream synthesis processes have been disclosed by Hashimoto, et al. [U.S. Application Publication No. 2010/0285657], by Letts, et al. [U.S. Application Publication No. 2010/0126411], and by Kubota, Saito, and Nagaoka [Japanese Patent Application Publication Nos. 2014-118327, 2014-139118, 2014-189426, and 2014-227314]. D'Evelyn, et al. [U.S. Pat. Nos. 8,461,071 and 8,987,156] disclosed the deliberate addition of a getter composition to a polycrystalline group III nitride.
These methods, whether conversion-in-place, powder, or downstream synthesis, suffer from various shortcomings, as described further below. What is needed is a method for scalable, efficient, low-cost manufacturing of polycrystalline nitride materials that are suitable for crystal growth of bulk gallium nitride crystals and do not contribute excess impurities to the bulk crystals. The present invention fulfills this need among others.